In a nuclear reactor, the fissionable material gradually becomes spent and must be removed. Since the spent fuel contains fission by products which are highly radioactive, and which generate large amounts of heat, the spent fuel is usually temporarily stored in the reactor's spent fuel pool. The spent fuel pool is a pool of water of sufficient volume to prevent the escape of harmful radiation, and to absorb and dissipate the heat generated by the decaying fissionable material. Alternatively, the spent fuel may be temporarily stored in a hot cell. That is, a heavily shielded structure having the capability to prevent the escape of harmful radiation, while absorbing and dissipating the heat generated by the spent fuel.
Generally, there is limited storage space in a nuclear reactor's spent fuel pool, or in its hot cell. Thus, the spent fuel must be moved to a storage site to make room for additional spent fuel. In some cases, there is a desire to shut the nuclear reactor down, and remove all fissionable material, in which case, all of the fissionable material must be removed to a storage site.
There are two primary problems in the transportation of spent fuel. The most difficult problem is the transportation of spent fuel that includes failed fuel rod assemblies. Typically, nuclear fuel is formed of numerous small pellets that are inserted into a hollow rod. In some cases the rods become damaged and allow some of the nuclear fuel pellets to escape. These damaged rods are known as failed fuel rods. Further, in some cases during nuclear reaction of the fuel, the pellets disintegrate into sand-sized particles, capable of easily escaping from a failed fuel rod. The fuel rods themselves are arranged into assemblies including several fuel rods. Thus, a fuel rod assembly including a failed fuel rod is termed a failed fuel rod assembly.
An important part of transporting and storing spent fuel is avoiding criticality. This is achieved by carefully arranging the spent fuel rod assemblies so that there is a minimum distance between each assembly, such that there is little chance of neutron multiplication occurring to the point of criticality. In the case of failed fuel rod assemblies, however, fissionable material can escape from failed rods, and potentially accumulate near enough to other fissionable material that criticality is achieved.
One attempted solution to the foregoing problem has been simply to store failed fuel rod assemblies indefinitely in a nuclear reactor's spent fuel pool or hot-cell. The problem with storing failed fuel rod assemblies indefinitely, however, is that there is limited storage space in a nuclear reactor's spent fuel pool or in its hot-cell, and in some cases there is a desire to completely shut a nuclear reactor down, and remove all fissionable material, including that contained in failed fuel rod assemblies.
Another attempted solution has been to transport failed fuel rod assemblies in fuel transportation containers designed for undamaged fuel rod assemblies. The foregoing attempted solution, however, has required that substantially fewer failed fuel rod assemblies be transported per container, compared to the number of undamaged fuel rod assemblies that can be transported in the same container. By transporting fewer failed fuel rod assemblies, even if some fissionable material escapes from the failed fuel rods, and accumulates near other fissionable material in the container, there is not enough fissionable material in the entire container to pose a significant risk of criticality. The problem with the foregoing solution, though, is it wasteful of resources, because significantly fewer failed fuel rod assemblies can be transported per container, relative to the number of undamaged fuel rod assemblies that can be transported in the same container.
Another, attempted solution has been to transport failed fuel rod assemblies in fuel transportation containers designed for transporting fissionable material in the form of rubble. That is, the fissionable material is not in the form of rods, but is in the form of small particles. Thus, the failed fuel rods are broken up into rubble, and placed in the container. The problem with that solution, however, is that the method is inefficient for three principle reasons. First, the failed fuel rod assemblies be broken up. Second, such containers are capable only of transporting comparatively few failed fuel rod assemblies. Finally, the transportation container is only designed for transportation, not storage. Thus, once the fissionable material has been transported to another location, the container must be unloaded in a fuel pool or in a hot cell, and other arrangements made to store the fissionable material.
The present invention solves the foregoing problems, and provides a device for transporting and storing failed fuel rod assemblies at a storage site, other than in a spent fuel pool or hot cell.
The other major problem with transporting spent nuclear fuel is that United States law imposes stringent safety requirements even on containers used to transport undamaged fuel rod assemblies. The relevant law imposes significantly more restrictive requirements with respect to the transportation of spent nuclear fuel across areas accessible to the public, as opposed to areas inaccessible to the public.
State of the art spent fuel transportation containers for areas accessible to the public are casks with individual compartments. The fuel rod assemblies are loaded into individual compartments in the casks in a spent fuel pool or a hot cell. The purpose of the individual compartments within each cask is to ensure sufficient spacing between adjacent fuel rod assemblies to avoid any danger of criticality. The fuel rod assemblies are loaded into the cask in a spent fuel pool or hot cell. Upon reaching the storage location, the fuel rod assemblies must be removed from the cask in a spent fuel pool or hot cell, and then stored.
In contrast, state of the art spent fuel transportation containers for areas inaccessible to the public are typically a sealed canister placed within a cask. The fuel rod assemblies are loaded into individual compartments in a canister in a spent fuel pool or a hot cell. The canister is then sealed and placed in a cask. When the cask/canister assembly reaches the storage site, the canister is removed from the cask, stored, and the cask may be reused, which is a much more efficient process.
Nonetheless, the cask/canister method cannot be used for transportation in areas accessible to the public because they fail to meet the requirements imposed by U.S. law. Accordingly, there is a need for an invention that provides for the transportation and storage of failed fuel rod assemblies, and for a cask/canister device for transportation and storage of spent fuel across areas accessible to the public. The present invention provides a solution, wherein a cask/canister device can be used, and additionally may be used with existing casks, resulting in much greater efficiency in the transportation over public thoroughfares and storage of spent nuclear fuel.